Viscosified treatment fluids comprising polycarboxylic acid gelling agents and associated methods

ABSTRACT

In one embodiment, the present invention provides a fluid comprising: an aqueous base fluid; and a gelling agent that comprises a polymerizable polycarboxylic acid and that is at least partially crosslinked by a crosslinking reaction comprising a crosslinking agent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to HES 2004-IP-014538U1, application Ser.No. ______ filed on the same day herewith.

BACKGROUND

The present invention relates to fluids useful as subterraneanviscosified treatment fluids, and more particularly, to novelpolycarboxylic acid copolymer gelling agents and viscosified treatmentfluids comprising these gelling agents, and their associated methods ofuse and manufacture.

As used herein, the term “treatment fluid” refers to any fluid that maybe used in a subterranean application in conjunction with a desiredfunction and/or for a desired purpose. The term “treatment fluid” doesnot imply any particular action by the fluid or any component thereof.Viscosified treatment fluids are used in a variety of operations insubterranean formations. For example, viscosified treatment fluids havebeen used as drilling fluids, fracturing fluids, and gravel packingfluids. Viscosified treatment fluids generally have a viscosity that issufficiently high, for example, to suspend particulates for a desiredperiod of time, to transfer hydraulic pressure, and/or to preventundesired leak-off of fluids into a formation. Viscosified treatmentfluids are often used in fracturing operations. Hydraulic fracturing isa technique for stimulating the production of desirable fluids from asubterranean formation. The technique normally involves introducing aviscosified treatment fluid through a well bore into a formation at achosen rate and pressure to enhance and/or create a fracture in aportion of the formation, and placing proppant particulates in theresultant fracture, inter alia, to maintain the fracture in a proppedcondition when the pressure is released. The resultant propped fractureprovides a conductive channel in the formation for fluids to flow to thewell bore. The degree of stimulation afforded by the hydraulicfracturing treatment is largely dependent on the conductivity and widthof the propped fracture.

Viscosified treatment fluids (e.g., fracturing fluids) that are used insubterranean operations generally are aqueous-based fluids that comprisea gelling agent, which may be crosslinked. These gelling agents may bebiopolymers or synthetic polymers. Common biopolymer gelling agentsinclude, e.g., galactomannan gums, cellulosic polymers, and otherpolysaccharides. Because of their cost and effectiveness, biopolymersare most commonly used. However, in high temperature applications, thesegelling agents can degrade, which can cause the viscosified treatmentfluid to prematurely lose viscosity. Various synthetic polymer gellingagents have been developed for use in viscosified treatment fluids.While some synthetic polymers have achieved some success, there arecontinuing needs for improved synthetic gelling agents that are stableat relatively high temperatures (e.g., above 300° F.).

The viscosity of a viscosified treatment fluid may be increased bycrosslinking the gelling agent in the fluid. Gelling agent molecules aretypically crosslinked through a crosslinking reaction with acrosslinking agent that has been added to the treatment fluid. Thesecrosslinking agents generally comprise a metal, transition metal, ormetalloid, collectively referred to herein as “metal(s).” Examplesinclude boron, aluminum, antimony, zirconium, magnesium, or titanium.Generally, the metal of a crosslinking agent interacts with at least twogelling agent molecules to form a crosslink between them. Under theappropriate conditions (e.g., pH and temperature), the crosslinks thatform between gelling agent molecules may increase the viscosity of aviscosified treatment fluid.

One type of fracturing fluid has been developed for use in deep, hightemperature wells comprises a crosslinked synthetic gelling agent. Thegelling agent is a high molecular weight copolymer of 60% to 78%acrylamide and 22% to 40% potassium acrylate crosslinked with a titaniumor zirconium compound. In preferred embodiments, the copolymer comprisesabout 30% of the acrylate monomer to achieve optimal crosslinking.Unfortunately, an acrylate concentration this high may cause the gellingagent to have poor salt tolerance in subterranean applications, andtherefore, these fluids are not optimal in higher pressure andtemperature deep wells.

Viscous foamed treatment fluids have been used in conventionalfracturing operations. Benefits of using foamed treatment fluids in suchoperations include a reduced risk of: leak-off of the fluid into apermeable formation, and damage to the formation by polymer residuedeposits. Also, because a foamed treatment fluid is less dense than aconventional treatment fluid, flow back of the fluid may occur.

The gases typically used in forming foamed treatment fluids usually arenitrogen, carbon dioxide, and mixtures thereof. Generally carbon dioxideis more economical for wells having greater depths and correspondinglyhigher pressures and temperatures. Carbon dioxide can be pumped at alower wellhead pressure than nitrogen because it has a higher densitythan nitrogen at similar conditions.

Carbon dioxide foamed fracturing fluids heretofore have been utilized insubterranean zone having temperatures up to about 400° F. However, theviscosity of a foamed fracturing fluid is at least partially dependentupon the liquid phase thereof, and the viscous liquids utilizedheretofore have generally been unstable at temperatures above about 300°F. Aqueous gelled liquids containing gelling agents such as guar,hydroxypropylguar, and carboxymethylhydroxypropylguar lose viscosity bythermal thinning, and become hydrolytically unstable above about 350° F.Also, at 350° F. and above, the heretofore used carbon dioxide foamedtreatment fluids have not demonstrated desirable proppant particlecarrying capability.

SUMMARY

The present invention relates to fluids useful as subterraneanviscosified treatment fluids, and more particularly, to novelpolycarboxylic acid copolymer gelling agents and viscosified treatmentfluids comprising these gelling agents, and their associated methods ofuse and manufacture.

In one embodiment, the present invention provides a fluid comprising: anaqueous base fluid; and a gelling agent that comprises a polymerizablepolycarboxylic acid and that is at least partially crosslinked by acrosslinking reaction comprising a crosslinking agent.

In another embodiment, the present invention provides a gelling agentcomprising a copolymer that comprises: an AMPS monomeric unit; apolycarboxylic acid monomeric unit; and an acrylamide monomeric unit.

In another embodiment, the present invention provides a gelling agentthat comprises a polymerizable polycarboxylic acid and that is at leastpartially crosslinked by a reaction comprising a crosslinking agent anda polycarboxylic acid copolymer gelling agent.

The features and advantages of the present invention will be readilyapparent to those skilled in the art upon a reading of the descriptionof the preferred embodiments that follows.

BRIEF DESCRIPTION OF THE FIGURES

These drawings illustrate certain aspects of some of the embodiments ofthe present invention, and should not be used to limit or define theinvention.

FIG. 1 illustrates a graph of the results from experiments relating tothe performance of polymers containing itaconic acid or acrylic acid atvarious crosslinker concentrations.

FIG. 2 illustrates a graph of the results from experiments relating tothe performance of polymers containing 3-butene-1,2,3-tricarboxylic acidat 400° F.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to fluids useful as subterraneanviscosified treatment fluids, and more particularly, to novelpolycarboxylic acid copolymer gelling agents and viscosified treatmentfluids comprising these gelling agents, and their associated methods ofuse and manufacture. The viscosified treatment fluids of the presentinvention may be useful in any application requiring a viscosifiedtreatment. They are especially suitable for any subterranean applicationrequiring a viscosified treatment fluid, and they may be particularlysuited for use in, for example, fracturing applications. These fluidsmay be particularly useful in hard rock fracturing applicationsinvolving higher temperatures and or pressures, e.g., in deeper wells.In certain embodiments, the viscosified treatment fluids of the presentinvention are useful in subterranean applications involving fluidtemperatures of up to and including about 415° F. In certainembodiments, the viscosified treatment fluids of the present inventionare useful in subterranean applications involving a bottom holetemperature of up to and including about 450° F. Higher temperaturereservoirs may be treated with cooling pre-flushes if desired, e.g., toprevent possible damage to the fluid.

One of the advantages of the gelling agents of the present invention isthat they require less of the crosslinking agent than previous gellingagents to achieve the same viscosity effect. Using less crosslinkermeans that the fluid is potentially less damaging to the formation inthat less residue is deposited in the formation after the fluid breaks.The permeability of the formation is not undesirably negatively impactedby the gelling agent.

The viscosified treatment fluids of the present invention comprise anaqueous base fluid, and a gelling agent that comprises a polymerizablepolycarboxylic acid copolymer. Optionally, the gelling agent may be atleast partially crosslinked by a crosslinking reaction comprising acrosslinking agent. The viscosified treatment fluids also may comprise asuitable breaker, if desired. In alternative embodiments, theviscosified treatment fluids of the present invention may be foamed. Insuch embodiments, the viscosified treatment fluids also comprise a gas,and a foaming agent. Optionally, other additives suitable for use inconjunction with the viscosified treatment fluids of the presentinvention may be added if desired.

The gelling agents of the present invention comprise a polymerizablepolycarboxylic acid copolymer comprising the following three monomericunits: 2-acrylamido-2-methylpropanesulfonic acid or salts thereof(hereinafter referred to collectively as “AMPS”); a polymerizablepolycarboxylic acid or salts thereof (hereinafter referred tocollectively as “a polymerizable polycarboxylic acid”); with the balancebeing acrylamide. Suitable polymerizable polycarboxylic acids havevicinal carboxylic acid groups. Examples include itaconic acid, maleicacid, 3-butene-1,2,3-tricarboxylic acid, and fumaric acid. Also suitableare any precursors of these acids. The gelling agents of the presentinvention, generally speaking, hydrate in the presence of an aqueousfluid, and can be easily crosslinked by suitable metal ions.

In preferred embodiments, the monomeric units in a polycarboxylic acidcopolymer are arranged in random fashion rather than in a blockarrangement. A block arrangement may not be as desirable because it mayinhibit the ability of the gelling agent to crosslink.

In certain embodiments, the weight fraction of the AMPS in thepolycarboxylic acid copolymer may be from about 15% to about 80% byweight (preferably about 60%) of the copolymer, the weight fraction ofthe polymerizable polycarboxylic acid may be from about 0.1% to about 1%by weight (preferably about 0.1% to 0.5%) of the copolymer, andacrylamide may comprise the balance of the weight of the copolymer.

Formulae 1-3 below illustrate some embodiments of the polymerizablepolycarboxylic acid copolymers of the gelling agents of the presentinvention:

Salts of these acids may also be appropriate.

Preferably, the polycarboxylic acid copolymers of the gelling agents ofthe present invention may be at least partially crosslinked by acrosslinking reaction that comprises any suitable crosslinking agent.Preferred crosslinking agents include zirconium-based crosslinkingagents and titanium-based crosslinking agents. Hafnium also may besuitable. Zirconium-based commercially available crosslinking agentssuitable for use in this invention include those available under thetrade names “CL-23” and “CL-24,” which are both available fromHalliburton Energy Services in Duncan, Okla. Boron-based crosslinkingagents are not preferred.

Importantly, in the preferred embodiments, less of the crosslinkingagent than previously thought to have been needed is required when usingthe gelling agents of the present invention to achieve a given viscosityfor a viscosified treatment fluid. In certain embodiments, thecrosslinking agent may be included in a viscosified treatment fluid inan amount in the range of from about 0.02% to about 1.2% by volume ofthe aqueous base fluid, more preferably in the amount of about 0.5%.

The aqueous base fluid may include fresh water, or water containing oneor more salts dissolved therein. Fresh water or a 2% KCl brine may bepreferred. The water can be from any source, as long as it does notcontain an excess of compounds that adversely affect other components inthe treatment fluid. Potentially problematic ions include divalent andtrivalent ions that may form complexes with the gelling agent that mayimpede crosslinking. These potentially problematic ions include calcium,iron, aluminum, and magnesium.

In embodiments wherein the viscosified treatment fluids of the presentinvention are foamed, gases and foaming agents may be included in thefluids. While various gases can be utilized for foaming the viscousaqueous fracturing fluids of this invention, nitrogen, carbon dioxide,and mixtures thereof are preferred. In examples of such embodiments, thegas may be present in a viscosified treatment fluid in an amount in therange of from about 5% to about 95% by volume of the treatment fluid,and more preferably in the range of from about 20% to about 70%. Theamount of gas to incorporate into the fluid may be affected by factorsincluding the viscosity of the fluid and wellhead pressures involved ina particular application. Examples of preferred foaming agents that canbe utilized to foam and stabilize the viscosified treatment fluids ofthis invention include, but are not limited to, C₈ to C₂₂alkylamidobetaines such as cocoamidopropyl betaine, alpha-olefinsulfonate, trimethyltallowammonium chloride, C₈ to C₂₂ alkylethoxylatesulfate and trimethylcocoammonium chloride. Cocoamidopropyl betaine isespecially preferred. Other suitable foaming agents and foam stabilizingagents may be included as well, which will be known to those skilled inthe art with the benefit of this disclosure. The foaming agent isgenerally present in a viscosified treatment fluid of the presentinvention in an amount in the range of from about 0.1% to about 2.0% byweight, more preferably in the amount of from about 0.2% to about 1.0%and most preferably about 0.6%.

In some applications, after a viscosified treatment fluid has performedits desired function, its viscosity may be reduced, which often isreferred to as “breaking” the viscosified treatment fluid Breaking aviscosified treatment fluid may occur naturally over time or, e.g., byadjusting the pH of the viscosified treatment fluid. Breaking aviscosified treatment fluid typically involves incorporating suitable“breakers” in the viscosified treatment fluid. For the viscosifiedtreatment fluids of the present invention, suitable breakers includeoxidizers and acids. Because acids may not achieve sufficient viscosityreduction in certain circumstances, oxidizers are generally preferred.In certain embodiments, the breaker may be formulated or presented in amanner in which it may act in a delayed fashion. Examples of suchdelayed breakers include, but are not limited to, encapsulated breakers,(e.g., encapsulated oxidizers). Alternatively, any of the delayedbreakers conventionally used with metal ion crosslinking agents may beused, for example, oxidizers such as sodium chlorite, sodium bromate,sodium persulfate, ammonium persulfate, encapsulated sodium persulfate,potassium persulfate, or ammonium persulfate and the like as well asmagnesium peroxide. One should note that persulfates may break atreatment too quickly at high temperatures. Sodium bromate is apreferred breaker. The specific breaker used, whether or not it isencapsulated, and the amount thereof employed, will depend upon thebreaking time desired, the nature of the gelling agent and crosslinkingagent, formation characteristics and conditions, and other factorsknown, with the benefit of this disclosure, to individuals skilled inthe art.

In certain embodiments, the viscosified treatment fluids of the presentinvention may comprise particulate materials like proppant or gravelthat can be utilized in, for example, subterranean applications likefracturing operations. Suitable particulate materials include, but arenot limited to, graded walnut or other nut shells, resin-coated walnutor other nut shells, graded sand, resin-coated sand, sintered bauxite,various particulate ceramic materials, glass beads, various particulatepolymeric materials, and the like. The particular size of theparticulate material employed may depend on the particular applicationfor which the particulate materials are being used, characteristics ofthe subterranean formation, and characteristics of the particular gellednonaqueous treatment fluid being used, as well as other variables.Generally, the sizes of suitable particulates may vary in the range offrom about 2 mesh to about 200-mesh, U.S. Sieve Series scale. Theparticulates may be coated with a curable resin and/or a tackifier ifdesired. One of ordinary skill in the art, with the benefit of thisdisclosure, will be able to choose an appropriate particulate materialfor a given application.

Additional additives may be present in the viscosified treatment fluidsof the present invention as deemed appropriate by one skilled in the artwith the benefit of this disclosure. Examples of such additives include,but are not limited to, surfactants, scale inhibitors, clay stabilizers,gases, antifoaming agents, foaming agents, storage stabilizers,biocides, biostatic agents, or combinations thereof. An example of anadditive that may be included is a pH buffer. In certain embodiments,the pH of a viscosified treatment fluid should be maintained in therange of from about 4 to about 6. While various buffers may be suitable,a particularly suitable buffer is acetic acid-acetate. When used, thebuffer should be included in a viscosified treatment fluid in an amountin the range of from about 0.1% to about 1.0% v/v.

An embodiment of a method of making the viscosified treatment fluids ofthe present invention comprises the steps of: combining and a gellingagent that comprises a polymerizable polycarboxylic acid and an aqueousbase fluid so that the gelling agent hydrates in the aqueous base fluid;adjusting the pH of the fluid to a range of from about 4.5 to about 7 (5to 6 may be preferred) with a suitable pH adjuster; and allowing aviscosified treatment fluid to form. Suitable pH adjusters include anypH adjuster that is compatible with the gelling agents of the presentinvention. Examples include buffers (e.g., an acetate buffer). Asuitable crosslinking agent may be added so that the gelling agent atleast partially crosslinks.

The viscosified treatment fluids of the present invention can beutilized for carrying out a variety of subterranean well treatments,including, but not limited to, fracturing operations. In certainexemplary embodiments wherein a viscosified treatment fluid is used inconjunction with fracturing operations, fracturing fluids comprising anaqueous base fluid and a gelling agent that comprises a polymerizablepolycarboxylic acid of this invention may be placed in a subterraneanformation at a sufficient pressure to create or enhance one or morefractures in a portion thereof. In some instances, the gelling agentwill be at least partially crosslinked by a reaction comprising asuitable crosslinking agent. After the fracturing fluid has performedits desired function, or after a desired period of time, the viscosityof the fracturing fluid may be reduced (e.g., by a breaker) and at leasta portion of the fracturing fluid may be recovered.

In one example of the methods of this invention, a viscosified treatmentfluid comprising a gelling agent that comprises a polymerizablepolycarboxylic acid that is at least partially crosslinked by a reactioncomprising a suitable crosslinking agent is placed into a subterraneanformation at a rate and pressure sufficient to create or enhance afracture therein. The viscosified treatment fluid may be broken to forma reduced viscosity treatment fluid, and then the reduced viscositytreatment fluid may be at least partially recovered at the surface. Inan alternative embodiment, the viscosified treatment fluid may befoamed.

To facilitate a better understanding of the present invention, thefollowing examples of preferred embodiments are given. In no way shouldthe following examples be read to limit, or define, the scope of theinvention.

EXAMPLES

Polymer Synthesis: Polymers were prepared from 10% w/w solutions ofmonomer in degassed, deionized water. Monomer composition was 60% w/wAMPS, x% w/w carboxylic acid monomer, and (40−x)% w/w acrylamide.Approximately 15 ppm of potassium persulfate was added as an initiator.The solutions were contained in tightly-capped, glass bottles. Bottleswere placed in a 65° C. water bath for four hours and then cooled toroom temperature.

Rheological Characterization at 400° F.: The 10% polymer solution wasdiluted to 0.75% with water while shearing in a Waring blender. Thesolution was neutralized to pH 5-6 with sodium hydroxide. Aceticacid/acetate buffer, if used, was added at the concentration indicated.Zirconium crosslinker (CL-23) was then added. Viscosity was measured at400° F. using a Fann model 50 viscometer with a B5X bob at 95 rpm (81sec⁻¹) and a preheated bath.

Rheological Characterization at 330° F.: The 10% polymer solution wasdiluted to 1% with water while shearing in a Waring blender. Thesolution was neutralized to pH 5-6 with sodium hydroxide. Zirconiumcrosslinker (CL-23) was added at the concentration indicated. Viscositywas measured at 330° F. using a Brookfield PVS viscometer with a B5 bobat 95 rpm (81 sec⁻¹).

Comparison of Itaconic Acid and Acrylic Acid at 330° F. The acrylic acidcopolymer composition used was: 0.5% acrylic acid, 60% AMPS, and 39.5%acrylamide. The itaconic acid copolymer composition used was: 0.45%itaconic acid, 60% AMPS, and 39.55% acrylamide. The concentrations ofcarboxylic acid monomers were selected to give polymers with the sameconcentration of carboxylate groups. 1% solutions of the polymers wereadjusted to pH of about 5.3 with sodium hydroxide, and CL-23 crosslinkerwas added in the concentrations shown in FIG. 1. High viscosity isachieved with the itaconic acid copolymer using a crosslinkerconcentration of 6 gal/Mgal. To obtain similar performance with theacrylic acid copolymer, twice the amount of crosslinker (12 gal/Mgal) isrequired. If 12 gal/Mgal of crosslinker is used with the itaconic acidcopolymer, overcrosslinking and viscosity degradation appears to occur.Good viscosity is obtained from the itaconic acid copolymer with only 3gal/Mgal of crosslinker, exceeding the viscosity of the acrylic acidcopolymer with 6 gal/Mgal of crosslinker.

Comparison of 3-Butene-1,2,3-tricarboxylic Acid (BTCA) Copolymers at400° F. Polymers were prepared that contain 0.1% or 0.5% BTCA, 60% AMPS,and acrylamide, q.s. 0.75% solutions of the polymers were adjusted to pH5-6 with sodium hydroxide and 3 gal/Mgal acetic acid/acetate buffer wasadded. Each solution was crosslinked with 3 gal/Mgal of CL-23 zirconiumcrosslinker. Good performance was observed with the polymer containing0.1% BTCA (see FIG. 2). The higher concentration of BTCA provides highermaximum viscosity, but shear degrades more quickly due to highercrosslinking density.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Whilenumerous changes may be made by those skilled in the art, such changesare encompassed within the spirit of this invention as defined by theappended claims, in which the terms used should be construed to havetheir plain, ordinary meaning except where specifically defined by thepatentees.

1. A fluid comprising: an aqueous base fluid; and a gelling agent thatcomprises a polymerizable polycarboxylic acid copolymer, and that is atleast partially crosslinked by a crosslinking reaction comprising acrosslinking agent.
 2. The fluid of claim 1 wherein the polycarboxylicacid copolymer gelling agent comprises at least one componentcorresponding to one of these formulae below:


3. The fluid of claim 1 wherein the polycarboxylic acid copolymergelling agent comprises a polycarboxylic acid copolymer that comprisesthe following three monomeric units: AMPS; a polymerizablepolycarboxylic acid; and acrylamide.
 4. The fluid of claim 3 wherein themonomeric units are arranged in a random fashion.
 5. The fluid of claim3 wherein the weight fraction of AMPS in the copolymer is from about 15%to about 80% be weight; the weight fraction of the polymerizablepolycarboxylic acid in the copolymer is from about 0.1% to about 1.0% byweight; and the acrylamide comprises the balance of the weight of thecopolymer.
 6. The fluid of claim 1 wherein the polycarboxylic acidcopolymer gelling agent comprises a polymerizable polycarboxylic acidthat comprises vicinal carboxylic acid groups.
 7. The fluid of claim 1wherein the polycarboxylic acid copolymer gelling agent comprises atleast one of the following: itaconic acid; maleic acid; or3-butene-1,2,3-tricarboxylic acid; or a derivative thereof.
 8. The fluidof claim 1 wherein the crosslinking agent comprises at least one of thefollowing: a zirconium-based crosslinking agent, a titanium-basedcrosslinking agent, or a hafnium-based crosslinking agent.
 9. The fluidof claim 1 wherein the crosslinking agent is present in the fluid in anamount of from about 0.02% to about 1.2%.
 10. The fluid of claim 1wherein the fluid comprises at least one of the following: a breaker; agas; a foaming agent; fresh water; water comprising a salt; a brine; aKCl brine; a particulate material; a surfactant; a scale inhibitor; aclay stabilizer; an antifoaming agent; a storage stabilizer; a biocide;a buffer; or a biostatic agent; or a derivative thereof.
 11. The fluidof claim 10 wherein the gas is present in an amount of from about 5% toabout 95% by volume of the fluid.
 12. The fluid of claim 1 furthercomprising a gas present in an amount of from 5% to about 95% by volumeof the fluid, and a foaming agent present in an amount of from about0.1% to about 2% by volume.
 13. A gelling agent comprising a copolymerthat comprises: an AMPS monomeric unit; a polycarboxylic acid monomericunit; and an acrylamide monomeric unit.
 14. The fluid of claim 13wherein the monomeric units are arranged in a random fashion in thecopolymer.
 15. The fluid of claim 13 wherein the weight fraction of AMPSin the copolymer is from about 15% to about 80% be weight; the weightfraction of the polymerizable polycarboxylic acid in the copolymer isfrom about 0.1% to about 1.0% by weight; and the acrylamide comprisesthe balance of the weight of the copolymer.
 16. The fluid of claim 13wherein the polycarboxylic acid copolymer gelling agent comprises apolymerizable polycarboxylic acid that comprises vicinal carboxylic acidgroups.
 17. The fluid of claim 13 wherein the polycarboxylic acidcopolymer gelling agent comprises at least one of the following:itaconic acid, maleic acid, 3-butene-1,2,3-tricarboxylic acid; and aderivative thereof.
 18. A gelling agent that comprises a polymerizablepolycarboxylic acid and that is at least partially crosslinked by areaction comprising a crosslinking agent and a polycarboxylic acidcopolymer gelling agent.
 19. The fluid of claim 18 wherein thecrosslinking agent comprises at least one of the following: azirconium-based crosslinking agent, a titanium-based crosslinking agent,or a hafnium-based crosslinking agent.
 20. The fluid of claim 18 whereinthe polycarboxylic acid copolymer gelling agent comprises a copolymerhaving the following three monomeric units: AMPS; a polymerizablepolycarboxylic acid; and acrylamide.